When LEDs (light emitting diodes) replace traditional light sources, such as incandescent sources, there is often a desire to have the LEDs produce light and lighting effects similar to those produced by traditional light sources. Dimming is one example of this. An incandescent source is a single source point which begins to dim up from the center. As the brightness increases the single source point becomes brighter giving the effect of center to edge brightness. In contrast to an incandescent fixture, LED fixtures typically light up from multiple source points. A typical LED fixture includes an AC to DC driver and a number of LEDs arranged in parallel and serial strings. As the driver increases its output current, all of the LEDs begin to emit photons simultaneously and increase in unison until full brightness is achieved.
In order to achieve center to edge brightness in LED fixtures some fixtures use multiple output drivers to control the output in stages so that first the center LED string, then the edge LED strings are lit. However, a fixture with multiple output drivers and the associated controls needed to control the drivers result in a complex and costly design. Thus, there is a need for a more cost effective approach to provide center to edge brightness in an LED fixture.
Another difference between incandescent sources and LED sources is the way the color temperature changes as the light level increases. Incandescent sources generate light by the glowing of a metal, such as tungsten. The color temperature of the glowing element is low at low light levels and progressively increases as the light level increases. LEDs do not change color temperature in the same manner as an incandescent source. In order to achieve the color temperature change of an incandescent source, some LED fixtures use multiple output drivers and controls to drive LEDs of different color temperatures at different times. However, this approach is both costly and complex. Thus, there is a need for a more cost effective approach for providing color temperature change as light levels increase in an LED fixture.